The present invention is in the field of battery technology and, more particularly, electrolyte formulations that address challenges encountered during the use of silicon anodes in lithium ion batteries.
Lithium ion batteries enjoy relatively widespread use, but research continues into improving the energy density, capacity, and cycle life of these batteries. For example, silicon has been used as an anode material to improve the energy density of lithium ion cells. Silicon anodes can provide high energy density to lithium ion batteries due to the high theoretical capacity of silicon, which is 4200 mAh/g. However, the silicon particles that make up the anode can undergo larges changes in their volume during battery cycling. The volumetric changes on lithiation and delithiation cycles can be as large as about 300%.
These large volumetric changes in the silicon anode material can have negative effects on battery cycle life. A number of mechanisms may contribute to poor cycle life. For example, silicon particles can fracture due to the large stresses in the material brought on by the large changes in volume during cycling. These fractures can result in electrically isolated particle fragments that can no longer contribute to the capacity during cycling. Even when silicon particles do not completely fracture, the large stresses in the anode material can result in cracks in the particle and delamination of the particle surface. These cracks and delaminations can result in portions of the active material being electrically isolated and unable to contribute to the capacity during cycling.
As another example of a failure mechanism, the solid-electrolyte interphase (SEI) that forms on the surface of silicon anode particles tends to not be mechanically robust. The result is cracking and delamination of this thin SEI layer on the particles as the large volume changes occur. Therefore, more SEI must be formed on each cycle to replace the cracked or delaminated SEI. But, this is not ideal because forming SEI irreversibly consumes battery capacity and creates gas products. Generally, a stable SEI should be formed on the initial cycles and should not need to be reformed.
Thus, there exists a need for an electrolyte formulation for silicon anodes in a lithium ion battery that improves cycle life by forming a more mechanically robust SEI. These and other challenges can be addressed by certain embodiments of the invention described herein.